The present invention relates to tires reinforced with metallic wires. The invention concerns in particular tires reinforced with stainless steel wires.
Patent application FR-A-2 096 405 describes a tire which comprises a reinforcement layer placed between the layers of the carcass and the rubber tread member, the radially exterior surface of this layer being reinforced by stainless steel metallic cables. No mention of using stainless steel in the carcass of tires is made in this application whose purpose is to prevent corrosion of the steel cables located in the crown of tires, this corrosion being due to humidity which reaches the cables through cracks and cuts of the rubber caused by aging or crossing over nails.
The applicant has found in an unexpected way that one could observe a decrease in the endurance of the steel cables in the carcass plies of tires, as a result of phenomena of fatigue and wear caused by the presence of water in the areas of the carcass, and this even in the absence of cuts or cracks of the rubber. The water which is responsible for this alteration of the properties of the cables probably penetrates into the areas of the carcass, coming from the outside, by migration through the rubber, for example from the inflation gas, or it was already found initially on the inside of the tire, owing to an important humidity content of the rubbers used or resulting from the curing. Lastly, this water can probably also result from incorporation of ambient air in the rubber, at the time of calendering of the cables.
The purpose of the invention is to remedy the above-noted disadvantages or to alleviate them to a large extent.
Consequently, the invention concerns a steel wire for the carcass of a tire characterized by the following points:
a) its diameter is at least equal to 0.05 mm and at most equal to 0.5 mm;
b) its tensile strength is at least equal to 2000 MPa;
c) the steel of the wire is stainless and it comprises at least 0.02% and at most 0.2% carbon, at least 3% and at most 20% nickel, at least 12% and at most 28% chromium, the sum of the nickel and chromium being at least equal to 20% and at most equal to 35%, all of these values in percentages being the percentages by weight;
d) the structure of the steel comprises at least 20% in volume of martensite and it is devoid of austenite or it comprises less than 80% of it by volume.
The invention also concerns the assemblies intended to be used in the carcasses of tires, these assemblies including at least one wire consistent with the invention.
The invention also concerns the carcasses of tires which include at least one wire or one assembly consistent with the invention.
The invention also concerns the tires which include at least one carcass consistent with the invention.
The invention also concerns a process for preparing the wire in accordance with the invention, this process being characterized by the following points:
a) one starts with a stainless steel wire whose diameter is at least equal to 0.3 mm and at most equal to 3 mm; the steel of the wire comprises at least 0.02% and at most 0.2% carbon, at least 3% and at most 20% nickel, at least 12% and at most 28% chromium, the sum of the nickel and chromium being at least equal to 20% and at most equal to 35%, all of the percentages being by weight; the structure of this steel is entirely austenitic or practically austenitic;
b) at least one work hardening treatment is done, without heat treatment, the total deformation ratio being at least equal to 1.5.